The present invention pertains to improvements in the emission control of environmentally harmful and regulated nitrogen oxides (NO.sub.x) which are produced in a variety of processes such as the combustion of fossil fuels. More particularly, the invention relates to an improved process for the selective catalytic reduction of nitrogen oxides to nitrogen.
Atmospheric pollution caused by NO.sub.x emissions has become a matter of growing global concern in recent years. Nitrogen oxides contribute to acid rain and photochemical smog, and can cause respiratory problems. It is now recognized that the ground-level ozone is formed in the atmosphere through a photochemical reaction not only from volatile organic compounds but also from oxides of nitrogen.
The main sources of NO.sub.x emissions in industrialized countries are transportation, electric utilities, and industrial boilers. Much of the NO.sub.x is a product of combustion of fossil fuels such as coal, oil or gas.
Stringent regulations on NO.sub.x emission control are currently being implemented in industrialized countries and the limit of NO.sub.x discharge into the environment is successively being revised to place increasingly effective control requirements with an ultimate goal of zero NO.sub.x emission. In California, for instance, emission limits of 9 ppm or less have been imposed for industrial boilers above approximately 5860 kw (20 million btu/hr.)
Due to these stringent regulations on NO.sub.x emissions, the development of an effective NO.sub.x control technology has gained importance in recent years. To date, the most effective technology for controlling NO.sub.x emissions is the selective catalytic reduction (SCR) of NO.sub.x. In this method, NO.sub.x (NO+NO.sub.2) are reduced by NH.sub.3 to N.sub.2 and H.sub.2 O, usually at 250.degree.-400.degree. C. over a catalyst. The following reactions occur: EQU 4NO+4NH.sub.3 +O.sub.2 .fwdarw.4N.sub.2 +6H.sub.2 O EQU 6NO.sub.2 +8NH.sub.3 .fwdarw.7N.sub.2 +12H.sub.2 O EQU 2NO.sub.2 +4NH.sub.3 +O.sub.2 .fwdarw.3N.sub.2 +6H.sub.2 O
Since usually over 80 vol. % of the NO.sub.x is in the form of NO, the first reaction is the most important. Selective catalytic reduction by NH.sub.3 requires an ammonia injection system and an ammonia storage system. A practical disadvantage of this process is that it requires a complex and expensive set-up for safely handling NH.sub.3 which is a hazardous chemical.
Known catalytic systems which are able to catalyze effectively the above NO.sub.x reduction reactions using NH.sub.3 are supported noble metals, supported base metal oxides and zeolites. Noble metal catalysts such as those based on Pt, Rh, Ru or Pd supported on Al.sub.2 O.sub.3 or other carriers, which are used widely in catalytic converters for automobile-exhaust NO.sub.x reduction, are usually not considered for flue gas treatment due to several drawbacks. These drawbacks include high cost, susceptibility to SO.sub.2 poisoning and substantial reduction of the catalytic activity at high temperatures or in the presence of excess oxygen due to accumulation of adsorbed oxygen.
Catalysts based on vanadia or tungsten-vanadia as active components supported on porous anatase-type titania are currently known to be most promising for the selective catalytic reduction of NO by NH.sub.3 mainly because of their high activity at low temperatures and good resistance to SO.sub.2 poisoning. These catalysts are presently used in many commercial installations. However, even with these catalysts, a number of problems are encountered. During the SCR process, NH.sub.3 can also undergo oxidation to undesirable NO.sub.x according to the following reactions: EQU 4NH.sub.3 +3O.sub.2 .fwdarw.2N.sub.2 +6H.sub.2 O EQU 4NH.sub.3 +5O.sub.2 .fwdarw.4NO+6H.sub.2 O EQU 2NH.sub.3 +2O.sub.2 .fwdarw.N.sub.2 O+3H.sub.2 O
When the NH.sub.3 oxidation proceeds in parallel with SCR, it results in a greater NH.sub.3 consumption and a lower NO.sub.x removal efficiency. Ammonia oxidation reactions are dominant at higher temperatures (&gt;425.degree. C.). The usual operating temperature required for SCR reaction ranges from about 300.degree. to about 425.degree. C. for peak NO.sub.x conversion efficiency. This temperature constraint limits the flexibility of the SCR reactor location in the integrated flue gas clean-up unit and incurs a heat exchanger cost for applications where the flue gas temperature exceeds this temperature limit. From a practical view point, the selectively and activity of the catalysts should be retained over a broad temperature range.
Another serious disadvantage with the selective catalytic reduction of NO.sub.x by NH.sub.3 is the risk of unacceptably high levels of ammonia emission known as "ammonia slip". The role of ammonia in polluting the atmosphere is well known. Ammonia slip can, in principle, be suppressed by lowering the reactor inlet NH.sub.3 /NO.sub.x ratio. This however, adversely affects the NO.sub.x removal efficiency.
Although vanadia and tungsten-vanadia based catalysts exhibit resistance to SO.sub.2 poisoning, they catalyze oxidation of SO.sub.2 to SO.sub.3. This latter compound (SO.sub.3) reacts with NH.sub.3 and H.sub.2 O to form compounds such as NH.sub.4 HSO.sub.4 and (NH.sub.4).sub.2 S.sub.2 O.sub.7. These compounds cause corrosion, plugging of the catalytic reactor and other parts of the system, and more undesirably, plugging of the pores of the catalysts. Pore plugging of the catalyst eventually results in a deactivation of the catalyst at a fixed NH.sub.3 /NO ratio and an increase of ammonia slip. The loss in activity can be restored by increasing the inlet NH.sub.3 /NO ratio. However, increasing the NH.sub.3 /NO ratio has the effect that ammonia slip also increases. Plugging of the catalyst pores and the reactor can also occur due to possible formation of NH.sub.4 NO.sub.3 by homogeneous reaction between NH.sub.3, NO.sub.2 and H.sub.2 O.